Wireless services as a paradigm for mobile computing and communication has gained momentum over the past decade. This has resulted in a host of techniques being proposed for modulating a signal such as FDMA, TDMA and CDMA for frequency, time and code division multiple access respectively. An essential element common to all the above transmitting and coding schemes is distribution of power in various urban, semi-urban, and indoor environments. With the rapid growth of wireless communication, the telecommunication industry is going through another revolution. The ever increasing demand for wireless services in urban and semi-urban areas has created large traffic density in these areas. To satisfy this, multiple transmitters have to be placed within small areas where high traffic densities occur. A simple approach would be to place many transmitters without regard to their locations, but this will increase co-channel and adjacent-channel carrier-to-interference ratios. Determination of `good` placement of transmitters is an iterative procedure which could be a very costly proposition if performed physically for every site where transmitters need to be placed. This cost increases much more for large outdoor urban and semi-urban environments because of the sheer logistics associated with such placement. Thus, a simulation which quickly computes radio frequency propagation in a geometrically represented model of an environment is highly desirable. Some other benefits of a simulation for a specific geographical site include useful signal propagation characteristics like bit error rate and delay spread profiles from various transmitter locations. Different transmitter characteristics (like power, gain patterns, frequency, etc,) can also be simulated with relative ease. Computation of delay spread includes recording power received by a receiving bin location from multiple paths.
Site specific modelling of urban environments is the representation of city and building environments as geometric databases whereby the buildings and cities are to scale with respect to their surroundings, though not necessarily with the finest geometric detail. A simulation where site specific information is either not available or not modeled is in the realm of statistical modeling. A statistical model for urban environment relies on broad classifications based on information like right angled street corners and very high buildings, but could not assume anything more specific than that. The obvious outcome is that predictions are less accurate. Considering that an urban environment, like downtown Manhattan, could be very complex, merely assuming that the streets are right angled does not suffice. Another method is empirical modeling which is a system requiring even less site specific information than a statistical model. In such a method, however, the prediction results are even less accurate.
In the art, there are a range of 2-dimensional simulation systems for radio propagation in site specific environments. For predicting RF propagation, many simple ray tracing techniques based site specific simulations have been used to compute average path loss and delay spread of the radio signals. One early proposed ray tracing based system which considers site specific geometric databases for propagation, traces paths from the receiver location, which severely restricts the number of receiving stations the system can simulate. In another system, building data with location, height and electrical properties of walls, etc., had been used for propagation prediction. Still other techniques use ray tracing for diffraction as well as for other modes of propagation and others have proposed site specific simulation for indoor propagation. Moreover, ray tracing based schemes have been devised for some radio resource management functions, like handover and channel allocation, and attempts have been made for analyzing the effect of moving from a line-of-sight (LOS) street to a non line-of sight (NLOS) street on the received signal, i.e. the effect of turning a corner in an outdoor environment.
More advanced ray splitting schemes have been proposed to maintain a minimum spatial resolution for rays at the time of its interaction with a wall. The ray splitting takes place at predetermined zonal boundaries. Since the splitting is maintained at a constant spatial resolution, the number of rays increases exponentially with distance traveled. Also, since the concentric surfaces of the zones are predetermined, they do not account for the incident angle a ray makes with the intersecting surface. This can introduce arbitrarily large error due to insufficient subdivision of a ray for large incident angles.
In pure 2-dimensional (2D) geometric representations, rays are only shot in a single plane. Thus, roof reflections (for indoor environments), or ground reflections will not be accounted for. Also, the transmitters and receivers have to be placed in the same plane, resulting in very restricted usage for such a system. To simulate a different height for a transmitter other than the receivers, height could be specifically stored with each geometric entity. Computational formulations for the propagation loss due to such a difference in heights have been derived. For instance, the model known as the two-ray model computes ground reflected rays for outdoor propagation along LOS path and attenuates power analytically rather than geometrically. However, the model fails to account for other reflections like wall reflected rays in urban environments that reach a receiver.
A small variant to the basic 2-D representation is where height information for the polygons defining the buildings is given, allowing extrusion in the third dimension. This is an effective way to represent geometry if the environment has only horizontal and vertical surfaces, but it cannot represent geometries like slanted roofs, cathedral ceilings, domes, and terrain environments.
Most prior art systems as described above for propagation of radio frequency have used some variation of boundary representations (Breps). With Breps, the geometry is defined in terms of vertices and their topological relationship with each other and how that forms edges and faces. This is fine for small environments, but is inefficient for very large geometric databases because the faces are spatially unorganized.
Given the limitations inherent in the 2-D geometric representation models, a full 3-D site specific modelling system that can represent any arbitrarily oriented geometry and simulate RF propagation in 3-D would be highly desirable.
Furthermore, a full 3-D site specific modelling system that can effectively model multipaths including both ground and wall reflected rays, and, that could account for diffraction that is not restricted to a single plane, e.g., around edges of a building or reflections from adjoining buildings which are comparatively higher than the building on which the transmitter is deployed, would be highly desirable.
Additionally, a full 3-D site specific modelling system that can represent any indoor, outdoor and terrain-type environments which have varying elevation at the ground level, e.g. hills, would be extremely desirable.
Moreover, a full 3-D site specific modelling system that is visually interactive to facilitate prediction of RF coverage from various RF transmitter locations would be extremely desirable.